Positive electrode active materials, secondary batteries and electrical devices

Abstract

This application discloses a positive electrode active material, a secondary battery, and an electrical device, belonging to the field of battery technology. The positive electrode active material includes a core, a first shell layer disposed on at least a portion of the surface of the core, and a second shell layer disposed on at least a portion of the surface of the first shell layer. The core includes a magnetic oxide, the first shell layer includes at least one of lithium transition metal phosphate and / or lithium transition metal oxide, and the second shell layer includes a carbon material. The positive electrode active material of this application is recyclable and can effectively improve the cycle performance and rate performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a positive electrode active material, a secondary battery, and an electrical device. Background Technology

[0002] As a key electrochemical energy storage device, secondary batteries have been widely used in many fields due to their advantages such as high capacity, no memory effect, and wide range of applications.

[0003] In recent years, with the rapid development of the new energy vehicle industry, higher technical requirements have been put forward for the charge and discharge rate performance and cycle stability performance of secondary batteries in order to effectively alleviate users' range anxiety and improve the overall driving experience.

[0004] Therefore, this application is submitted. Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of the existing technology and provide a positive electrode active material, a secondary battery and an electrical device, wherein the positive electrode active material is recyclable and can effectively improve the cycle performance and rate performance of the secondary battery.

[0006] To achieve the above objectives, a first aspect of this application provides a positive electrode active material, the positive electrode active material comprising a core, a first shell layer disposed on at least a portion of the surface of the core, and a second shell layer disposed on at least a portion of the surface of the first shell layer; The core comprises a magnetic oxide, the first shell comprises at least one of lithium transition metal phosphate and / or lithium transition metal oxide, and the second shell comprises a carbon material.

[0007] As an implementation of this application, at least one of the following (A) to (D) is satisfied: (A) The magnetic oxide includes at least one of iron(II,III) oxide, iron(III) oxide, iron-silicon alloy, and iron-nickel alloy; (B) The lithium transition metal phosphate includes at least one of lithium iron phosphate and lithium manganese iron phosphate; (C) The carbon material includes at least one of hard carbon, soft carbon, graphene oxide, reduced graphene, and carbon nanotubes; (D) The lithium transition metal oxide includes at least one of lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide.

[0008] As an implementation of this application, at least one of the following (a) to (c) is satisfied: (a) The average particle size of the magnetic oxide is 30~200 nm; (b) The thickness of the first shell layer is 100~2000 nm; (c) The thickness of the second shell is 3~30nm.

[0009] As an embodiment of this application, the carbon material satisfies: 0.8 ≤ I d / I g ≤2; I d The intensity of peak D in the Raman spectrum of the carbon material; I g The intensity of the G peak in the Raman spectrum of the carbon material is given.

[0010] As an embodiment of this application, the specific surface area of ​​the positive electrode active material is 30~260 m². 2 / g.

[0011] As an embodiment of this application, the particle size Dv50 of the positive electrode active material is 100~2230nm.

[0012] As an embodiment of this application, the carbon material further includes an element M, which includes at least one of N, O, F, Cu, Ti, Co, and Ni.

[0013] As an embodiment of this application, the mass percentage of element M in the carbon material is 0.1-3%.

[0014] As an embodiment of this application, based on the mass content of the positive electrode active material, the proportion of the magnetic oxide is 5-25%, the proportion of the lithium transition metal phosphate and / or lithium transition metal oxide is 75-95%, and the proportion of the carbon material is 0.1-3%.

[0015] A second aspect of this application provides a secondary battery comprising the aforementioned positive electrode active material.

[0016] A third aspect of this application provides an electrical device including the aforementioned secondary battery, wherein the secondary battery serves as the power supply for the electrical device.

[0017] The beneficial effects of this application are as follows: The positive electrode active material described in this application includes a core, a first shell layer disposed on at least a portion of the surface of the core, and a second shell layer disposed on at least a portion of the surface of the first shell layer; using the magnetic oxide as the core, the core is magnetic, giving the positive electrode active material recyclability. After the positive electrode active material is decommissioned, it can be quickly recovered through magnetic response, solving the problems of traditional positive electrode active materials being difficult to separate from other components and having low recovery efficiency. Simultaneously, the magnetic oxide core can also improve the structural stability of the positive electrode active material, avoiding lattice distortion, runaway redox reactions, and other reactions during charging and discharging; the first shell layer is the active material layer, The first shell layer is located between the core and the second shell layer. The second shell layer can serve as a protective layer for the first shell layer, thereby improving the structural stability of the first shell layer, effectively alleviating the volume stress caused by lithium insertion / extraction during charging and discharging, preventing cracking, pulverization, or agglomeration, maintaining the crystal structure integrity of the first shell layer material, thereby reducing the capacity decay rate, shortening the lithium ion migration distance, increasing the lithium ion diffusion rate, reducing electron transport resistance and polarization, and improving rate performance. The second shell layer can improve electrolyte wetting, reduce formation time, effectively improve the conductivity of the positive electrode active material, increase the lithium ion migration rate, suppress the occurrence of side reactions, effectively suppress gas generation, and effectively improve the cycle performance and rate performance of the secondary battery. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0020] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0021] As a core component of rechargeable batteries, the positive electrode active material affects the battery's energy density, cycle life, and safety performance. Currently, the mainstream positive electrode active materials include lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide. With the rapid development of the new energy vehicle industry, the number of power batteries installed in vehicles continues to rise. Correspondingly, the retirement cycle of these power batteries is gradually approaching. The recycling and disposal of a large number of retired rechargeable batteries has become a key bottleneck restricting the green and sustainable development of the new energy industry. If these retired batteries are not properly recycled and disposed of, it will not only cause a serious waste of valuable mineral resources, but the heavy metals and electrolytes they contain will also seep into the soil and water, causing irreversible damage to the ecological environment, and will also affect the healthy and orderly development of the new energy industry.

[0022] As a core component of rechargeable batteries, the positive electrode active material accounts for a significant proportion of the total cost of the battery and contains various valuable metal elements such as lithium, manganese, iron, phosphorus, nickel, cobalt, and aluminum. Therefore, the efficient recycling and resource reuse of the positive electrode active material contained in retired rechargeable batteries can not only effectively reduce the total life cycle cost of the batteries but also realize the circular utilization of mineral resources, resulting in significant economic, environmental, and social benefits. It possesses extremely high recycling value and promising industrial application prospects. Currently, the recycling technologies for positive electrode active materials in retired rechargeable batteries mainly rely on traditional pyrometallurgical and hydrometallurgical methods. However, both of these methods have significant technical limitations and cannot meet the demands for green, efficient, and low-cost industrial recycling.

[0023] Therefore, based on the above-mentioned technical problems, this application provides a positive electrode active material, which includes a core, a first shell layer disposed on at least a portion of the surface of the core, and a second shell layer disposed on at least a portion of the surface of the first shell layer; The core comprises a magnetic oxide, the first shell comprises at least one of lithium transition metal phosphate and / or lithium transition metal oxide, and the second shell comprises a carbon material.

[0024] The positive electrode active material described in this application includes a core, a first shell layer disposed on at least a portion of the surface of the core, and a second shell layer disposed on at least a portion of the surface of the first shell layer. The core is a magnetic oxide, which is magnetic, giving the positive electrode active material recyclability. After the positive electrode active material is decommissioned, it can be quickly recovered through magnetic response, solving the problems of traditional positive electrode active materials being difficult to separate from other components and having low recovery efficiency. Simultaneously, the magnetic oxide core has an attractive effect on transition metal ions, inhibiting the dissolution of transition metal ions to a certain extent, thereby improving the structural stability of the positive electrode active material and reducing reactions such as lattice distortion and runaway redox reactions during charging and discharging. The first shell layer... The first shell layer is an active material layer, located between the core and the second shell layer. The second shell layer serves as a protective layer for the first shell layer, thereby improving the structural stability of the first shell layer, effectively alleviating the volume stress caused by lithium insertion / extraction during charging and discharging, reducing cracking, pulverization, or agglomeration, maintaining the crystal structure integrity of the first shell layer material, thus reducing the capacity decay rate, shortening the lithium ion migration distance, increasing the lithium ion diffusion rate, reducing electron transport resistance and polarization, and improving rate performance. The second shell layer can improve electrolyte wetting, reduce formation time, effectively improve the conductivity of the positive electrode active material, increase the lithium ion migration rate, suppress the occurrence of side reactions, effectively suppress gas generation, and effectively improve the cycle performance and rate performance of the secondary battery.

[0025] In some embodiments, the magnetic oxide includes at least one of iron(II,III) oxide, iron(III) oxide, iron-silicon alloy, and iron-nickel alloy. In particular, the use of such magnetic oxides can further improve the magnetic responsiveness of the positive electrode active material, enabling it to be separated quickly and accurately under the action of an external magnetic field, reducing material loss during the recycling process, and enhancing the stability of the core structure.

[0026] In some embodiments, the lithium transition metal phosphate includes at least one of lithium iron phosphate and lithium manganese iron phosphate.

[0027] In some embodiments, the carbon material includes at least one of hard carbon, soft carbon, graphene oxide, reduced graphene, and carbon nanotubes. Such carbon materials have excellent conductivity, can construct a continuous electron transport network, reduce impedance and improve rate performance, and have good flexibility, which can buffer the volume change of the first shell, improve cycle stability, and effectively suppress gas generation, thereby effectively improving the cycle life and rate performance of the secondary battery.

[0028] In some embodiments, the lithium transition metal oxide includes at least one of lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide.

[0029] In some embodiments, the average particle size of the magnetic oxide is 30~200nm, for example, it can be 30nm, 40nm, 50nm, 60nm, 80nm, 100nm, 120nm, 150nm, 180nm, 200nm or any two of these values. By controlling the average particle size of the magnetic oxide within this range, the structural stability of the positive electrode active material can be effectively improved, the active material can be stably supported, and volume stress can be relieved.

[0030] In some embodiments, the thickness of the first shell layer is 100~2000nm, for example, it can be 100nm, 200nm, 400nm, 500nm, 600nm, 800nm, 1000nm, 1200nm, 1500nm, 1800nm, 2000nm or any two of these values. By controlling the thickness of the first shell layer within this range, the structural stability of the positive electrode active material can be effectively improved, sufficient active sites can be ensured, and ion migration resistance can be reduced, thus balancing cycle stability and rate performance.

[0031] In some embodiments, the thickness of the second shell layer is 3~30nm, for example, it can be 3nm, 5nm, 6nm, 8nm, 10nm, 12nm, 15nm, 18nm, 20nm or any two of these values. By controlling the thickness of the second shell layer within this range, the wetting effect of the electrolyte on the first shell layer can be improved, lithium ion penetration can be accelerated, ion migration resistance can be reduced, side reactions caused by the dissolution of the first shell layer and the release of lattice oxygen can be reduced, and the gas production of the secondary battery can be effectively reduced.

[0032] The method for testing the thickness of the first and second shell layers is as follows: the secondary battery is discharged at a constant current of 0.33C to 2.5V, the positive and negative electrode plates are separated, the positive electrode powder is scraped off with a ceramic knife, 0.01g±0.001g of the positive electrode powder is ground in a mortar for 5 minutes, and then the positive active material is attracted out with a magnet. Then it is dried in a vacuum drying oven for 1 hour. After drying, it is taken out and dispersed in an ethanol solvent. The thickness of the first and second shell layers is measured by TEM transmission electron microscopy.

[0033] In some embodiments, the carbon material satisfies: 0.8 ≤ I d / I g ≤2, for example, can be a range consisting of 0.8, 1, 1.2, 1.4, 1.5, 1.6, 1.8, 2, or any two of these values, by controlling the I of the carbon material. d / I gWithin this range, the carbon material has a suitable degree of graphitization and defect density, which not only ensures improved rate performance by enhancing electron conduction efficiency, but also strengthens the interfacial bonding with the first shell layer, alleviates volumetric stress, improves cycle stability, and further enhances the cycle performance and rate performance of the secondary battery.

[0034] I d The intensity of peak D in the Raman spectrum of the carbon material is given. Peak D is located at 1340 cm⁻¹ in the Raman spectrum. -1 ~ 1400cm -1 scope; I g The intensity of the G peak in the Raman spectrum of the carbon material is given. The G peak is located at 1580 cm⁻¹ in the Raman spectrum. -1 ~ 1650cm -1 scope.

[0035] In some embodiments, the specific surface area of ​​the positive electrode active material is 20~260 m². 2 / g, for example, could be 20m 2 / g、30m 2 / g、40m 2 / g, 50m 2 / g, 60m 2 / g、80m 2 / g, 100m 2 / g, 120m 2 / g, 150m 2 / g, 180m 2 / g、200m 2 / g、200m 2 / g、240m 2 / g, 250m 2 / g、260m 2 / g or any two of these values, by controlling the specific surface area of ​​the positive electrode active material within this range, can ensure sufficient electrolyte wetting, increase the utilization rate of active sites, improve rate performance, reduce the occurrence of side reactions, and effectively improve the cycle life of the secondary battery.

[0036] The specific surface area test method for the positive electrode active material is as follows: the secondary battery is discharged at a constant current of 0.33C to 2.5V, the positive electrode and negative electrode are separated, the positive electrode powder is scraped off with a ceramic knife, 0.01g±0.001g of the positive electrode powder is ground in a mortar for 5 minutes, then the positive electrode active material is attracted out with a magnet, and then dried in a vacuum drying oven for 1 hour. The specific surface area of ​​the positive electrode active material is tested using a specific surface area and pore size distribution analyzer, and the test standard is GB / T 19587-2017 Gas Adsorption BET Method.

[0037] In some embodiments, the particle size Dv50 of the positive electrode active material is 100~2230nm, for example, it can be 100nm, 133nm, 140nm, 150nm, 200nm, 300nm, 400nm, 500nm, 600nm, 800nm, 1000nm, 1200nm, 1500nm, 1800nm, 2000nm, 2200nm, 2230nm or any two of these values.

[0038] The particle size Dv50 of the positive electrode active material refers to the particle size corresponding to a cumulative volume distribution percentage of 50% for the positive electrode active material. The test method is as follows: the secondary battery is discharged at a constant current of 0.33C to 2.5V, the positive electrode and negative electrode are separated, the positive electrode powder is scraped off with a ceramic knife, 0.01g±0.001g of the positive electrode powder is ground in a mortar for 5 minutes, then the positive electrode active material is attracted out with a magnet, and then dried in a vacuum drying oven for 1 hour. The particle size Dv50 is obtained by testing with a laser particle size analyzer.

[0039] In some embodiments, the carbon material further includes an element M, which includes at least one of N, O, F, Cu, Ti, Co, and Ni. By doping the first shell with element M, the dopant element can stabilize the first shell. Element M can provide the electronic structure of the carbon material, enhance conductivity and hydrophilicity, improve ion / electron transport efficiency, and improve the interfacial bonding between the first and second shells, thereby alleviating volumetric stress and effectively improving cycle performance and rate performance.

[0040] In some embodiments, the mass percentage of element M in the carbon material is 0.1% to 3%, for example, it can be 0.1%, 0.2%, 0.4%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.2%, 2.5%, 2.8%, 3%, or any two of these values. The method for testing element M is as follows: the secondary battery is discharged at a constant current of 0.33C to 2.5V, the positive electrode and negative electrode are separated, the positive electrode powder is scraped off with a ceramic knife, 0.01g±0.001g of the positive electrode powder is ground in a mortar for 5min, and then the positive active material is attracted out with a magnet, followed by drying in a vacuum drying oven for 1h; after drying, it is taken out and dispersed in ethanol solvent, and the morphology of the positive active material is photographed by HRTEM (high resolution transmission electron microscopy). The content and distribution of element M can be obtained by EDS mapping (energy dispersive spectroscopy) at the second shell position.

[0041] In some embodiments, the M element includes N, Ti, and O.

[0042] In some embodiments, based on the mass content of the positive electrode active material, the proportion of the magnetic oxide is 4-25%, for example, it can be 4%, 5%, 6%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, or any two of these values. By controlling the proportion of the magnetic oxide within this range, it can be ensured that the core has sufficient magnetism to guarantee the recovery efficiency, effectively improve the structural stability of the positive electrode active material, and at the same time avoid excessive proportion from crowding out the space of the active material and reducing capacity and rate performance, thus balancing the recovery effect and electrochemical performance.

[0043] In some embodiments, based on the mass content of the positive electrode active material, the proportion of the first shell layer is 75-95%, for example, it can be 75%, 76%, 78%, 80%, 82%, 84%, 85%, 86%, 88%, 90%, 92%, 94%, 95% or any two of these values. By controlling the proportion of the first shell layer within this range, sufficient active sites can be provided to ensure the capacity and cycle stability of the secondary battery.

[0044] In some embodiments, based on the mass content of the positive electrode active material, the proportion of carbon material is 0.1% to 3%, for example, it can be 0.1%, 0.2%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 1.6%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, or any two of these values. By controlling the proportion of carbon material within this range, the conductivity of the positive electrode active material can be effectively improved, the ion diffusion rate can be increased, the structural stability of the first shell layer can be improved, and the dissolution of metal elements in the first shell layer can be reduced, effectively improving the rate performance and cycle performance of the secondary battery.

[0045] One embodiment of this application provides a secondary battery, including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including the aforementioned positive active material.

[0046] In this application, there is no particular limitation on the type of positive electrode current collector, which can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper.

[0047] There are no particular restrictions on the form of the positive electrode current collector. When the positive electrode current collector is a metallic material, it can be in the form of metal foil, metal cylinder, metal strip, metal plate, metal foil, metal mesh, stamped metal, foamed metal, etc. When the positive electrode current collector is a carbon material, it can be in the form of carbon plate, carbon film, carbon cylinder, etc.

[0048] In one embodiment, the positive electrode active material layer further includes a conductive agent and a binder.

[0049] In one embodiment, the secondary battery further includes a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer including a negative active material.

[0050] In this application, there are no particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or composite current collector, etc.

[0051] In one embodiment, the negative electrode active material can be natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, or spinel-structured lithium titanate Li4Ti5O. 12 At least one of Li-Al alloys and metallic lithium.

[0052] In one embodiment, the negative electrode active material layer further includes a conductive agent and a binder.

[0053] In one embodiment, there is no limitation on the type of conductive agent mentioned in this application, and any known conductive agent may be used.

[0054] In one embodiment, the conductive agent includes at least one of carbon materials such as acetylene black, needle coke, carbon nanotubes, and graphene.

[0055] In one embodiment, there is no limitation on the type of adhesive mentioned in this application, and any known positive electrode adhesive can be used.

[0056] In one embodiment, the adhesive includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, styrene-butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymer or its hydrogenation, ethylene-propylene-diene terpolymer, styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer, syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymer.

[0057] In the secondary battery mentioned in this application, a separator is usually provided between the positive and negative electrodes to prevent short circuits. There are no particular restrictions on the material and shape of the separator, as long as it does not significantly impair the effectiveness of this application.

[0058] In one embodiment, the diaphragm comprises a porous sheet-like or nonwoven material with excellent liquid retention properties. Materials for resin or glass fiber diaphragms include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, and polyethersulfone.

[0059] In one embodiment, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is at least one of polypropylene and polyethylene. The materials of the diaphragm described above can be used alone or in any combination.

[0060] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0061] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0062] In some embodiments, the type of electrolyte is not specifically limited. The electrolyte includes an electrolyte salt and an organic solvent, and the specific types of the electrolyte salt and organic solvent are not specifically limited and can be selected according to actual needs. The electrolyte may also include additives, and the type of additives is not particularly limited. These additives can be film-forming additives for the positive and / or negative electrodes, or additives that can improve certain battery performance, such as additives that improve the battery's high or low temperature performance.

[0063] This application does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0064] One embodiment of this application provides an electrical device including the secondary battery described above, wherein the secondary battery serves as the power supply for the electrical device.

[0065] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0066] The present application is further illustrated below with specific embodiments: Example 1 A method for preparing a secondary battery includes the following steps: (1) Preparation of positive electrode active material: S1. Place Fe3O4 powder (average particle size of 80nm) in a plasma reaction vessel. At 25°C, continuously and uniformly introduce ammonia gas into the vessel. While introducing NH3, set the plasma power to 800W and continue the treatment for 20 minutes. After the reaction is completed, allow it to cool naturally to room temperature to obtain Fe3O4-NH2 for later use.

[0067] S2. The active material lithium manganese iron phosphate (LiMnFePO4) (Dv50 particle size of 100nm) and 30% hydrogen peroxide solution are mixed evenly at a mass ratio of 1:12, added to the plasma reaction vessel, the plasma power is set to 550W, and the treatment is carried out for 20min. After filtration and drying, LiMnFePO4-COOH is obtained for later use.

[0068] S3. Fe3O4-NH2 and LiMnFePO4-COOH are reacted in concentrated sulfuric acid at 25℃ for 45 min at a mass ratio of 1:80 to obtain Fe3O4@LiMnFePO4. The mixture is then filtered, dried, and purified.

[0069] S4. Fe3O4@LiMnFePO4 and a 1% graphene oxide solution were mixed at a mass ratio of 1:1 and stirred in a water bath at 45°C for 5 hours. After the reaction was completed, the mixture was centrifuged and dried in an oven at 60°C for 24 hours. Then, it was calcined (held at 220°C in a nitrogen atmosphere for 1.5 hours) to obtain the positive electrode active material.

[0070] The parameter tables of the positive electrode active material in this embodiment are shown in Table 1 and Table 2.

[0071] (2) Preparation of positive electrode sheet: The positive active material prepared above is mixed with the binder polyvinylidene fluoride PVDF and the conductive agent acetylene black at a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) is added and stirred evenly under the action of a vacuum stirrer to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated on an aluminum foil with a thickness of 12μm. The electrode sheet coated with the positive electrode slurry is dried, cold-pressed and slit to obtain a positive electrode sheet with a size of 700mm×120mm.

[0072] (3) Preparation of negative electrode sheet: The negative electrode active material hard carbon, thickener sodium carboxymethyl cellulose, binder styrene-butadiene rubber and conductive agent acetylene black are mixed in a mass ratio of 95.7:0.8:2.5:1, deionized water is added, and a negative electrode slurry is obtained under the action of a vacuum stirrer; the negative electrode slurry is uniformly coated on a copper foil with a thickness of 4.5 μm; the coated electrode sheet is dried, cold pressed and cut to obtain the negative electrode sheet.

[0073] (4) Separator: The diaphragm is a 10μm PE diaphragm.

[0074] (5) Preparation of electrolyte: At room temperature, in a glove box filled with argon (H2O<1ppm, O2<1ppm), ethyl methyl carbonate (EMC), propylene carbonate (PC) and dimethyl carbonate (DMC) are mixed evenly in a volume ratio of 1:1:1 to obtain a mixed solvent. Then, lithium salt LiPF6 is added to the mixed solvent and stirred evenly to obtain the electrolyte.

[0075] (6) Assembly of secondary battery: The prepared positive electrode, separator and negative electrode are stacked in sequence, with the separator in the middle of the positive electrode and negative electrode. After winding, hot pressing and shaping, the tabs are welded to obtain the bare cell. The bare cell is placed in the outer packaging aluminum-plastic film and baked in an oven at 85±10℃ for 24h. The electrolyte prepared above is injected into the dried aluminum-plastic film, and the cell is allowed to stand, form, and be divided into capacities to complete the preparation of secondary battery.

[0076] Example 2 The difference between Example 2 and Example 1 is that in Example 2, Fe2O3 is used instead of Fe3O4.

[0077] Examples 3-5 The difference between Examples 3-5 and Example 1 is that the average particle size of the iron oxide powder is changed, thereby changing the average particle size of the core, to achieve the parameters in Tables 1-2.

[0078] Examples 6-8 The difference between Examples 6-8 and Example 1 is that the thickness of the first shell is changed by altering the amount of LiMnFePO4-COOH added in step S3, thereby achieving the parameters in Tables 1-2.

[0079] Examples 9-11 The difference between Examples 9-11 and Example 1 is that the type of the second shell is changed.

[0080] Examples 12-14 The difference between Examples 12-14 and Example 1 is that the thickness of the second shell is changed by altering the mass percentage of the graphene oxide solution or the stirring time in the water bath, thus achieving the parameters in Tables 1-2.

[0081] Example 15 The difference between Example 15 and Example 1 is that step S4 is different. In this example, step S4 is as follows: Fe3O4@LiMnFePO4 and a 1% graphene oxide solution are mixed at a mass ratio of 1:1. Then, 12 mL of 0.25 wt% ammonia solution is added and the mixture is stirred in a water bath at 45°C for 5 h. After the reaction is completed, the mixture is centrifuged and dried in an oven at 60°C for 24 h. Then, it is calcined (held at 220°C in a nitrogen atmosphere for 1.5 h) to obtain the positive electrode active material.

[0082] Examples 16-18 The difference between Examples 16-18 and Example 15 is that the mass concentration of the ammonia solution is changed to change the mass content of M, thus achieving the parameters in Tables 1-2.

[0083] Example 19 Example 19 differs from Example 1 in that step S4 is different. In this example, step S4 is as follows: Fe3O4@LiMnFePO4 and a 1% graphene oxide solution are mixed at a mass ratio of 1:1 and stirred in a water bath at 45°C for 5 hours. After the reaction is completed, the mixture is centrifuged and dried in an oven at 60°C for 24 hours. Then, it is calcined (held at 220°C in a nitrogen atmosphere for 1.5 hours). The calcined product is mixed with titanium chloride at a mass ratio of 10:1 and calcined (held at 200°C in an argon atmosphere for 2 hours) to obtain the positive electrode active material.

[0084] Example 20 The difference between Example 20 and Example 1 is that step S4 is different. In this example, step S4 is as follows: Fe3O4@LiMnFePO4 and a 1% graphene oxide solution are mixed at a mass ratio of 1:1. Then, 12 mL of 0.25 wt% ammonia solution is added and the mixture is stirred in a water bath at 45°C for 5 h. After the reaction is completed, the mixture is centrifuged and dried in an oven at 60°C for 24 h. Then, it is calcined (held at 220°C in a nitrogen atmosphere for 1.5 h). The calcined product and titanium chloride are mixed at a mass ratio of 10:1 and calcined (held at 200°C in an argon atmosphere for 2 h) to obtain the positive electrode active material.

[0085] Examples 21-22 The difference between Examples 21 and 22 and Example 1 is that the type of magnetic oxide is changed to achieve the parameters in Tables 1 and 2.

[0086] Examples 23-24 The difference between Examples 23 and 24 and Example 1 is that the type of active material in step S2 is changed to achieve the parameters in Tables 1 and 2.

[0087] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the preparation method of the positive electrode active material is different, and Comparative Example 1 does not contain a second shell layer.

[0088] Preparation of the positive electrode active material in this comparative example: S1. Place Fe3O4 powder in a plasma reaction vessel and continuously and uniformly introduce ammonia gas into the vessel at 25°C. At the same time as introducing NH3, set the plasma power to 800W and continue the treatment for 20 minutes. After the reaction is completed, allow it to cool naturally to room temperature to obtain Fe3O4-NH2 for later use.

[0089] S2. Mix lithium manganese iron phosphate (LiMnFePO4) and 30% hydrogen peroxide solution at a mass ratio of 1:12, add the mixture to a plasma reaction vessel, set the plasma power to 550W, and continue the treatment for 20 minutes. Filter and dry to obtain LiMnFePO4-COOH for later use.

[0090] S3. Fe3O4-NH2 and LiMnFePO4-COOH are reacted in concentrated sulfuric acid at 25℃ in a mass ratio of 1:80. After filtration and drying, the positive electrode active material is obtained.

[0091] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the preparation method of the positive electrode active material in Comparative Example 2 is different from that in Example 1, and Comparative Example 2 does not contain the core of Example 1. The preparation method of the positive electrode active material in this comparative example is as follows: Lithium manganese iron phosphate (LiMnFePO4) (Dv50 particle size of 100nm) and graphene oxide solution with a mass fraction of 1% are mixed at a mass ratio of 1:1, stirred in a water bath at 45℃ for 5h, centrifuged after the reaction is completed, dried in an oven at 60℃ for 24h, and then calcined (held at 220℃ in a nitrogen atmosphere for 1.5h) to obtain the positive electrode active material.

[0092] Comparative Example 3 The difference between Comparative Example 3 and Example 1 lies in the preparation method of the positive electrode active material. Comparative Example 3 uses lithium manganese iron phosphate as its core and iron(III) oxide as its first shell. The specific preparation method is as follows: S1. Mix lithium manganese iron phosphate (LiMnFePO4) (Dv50 particle size of 100nm) and 30% hydrogen peroxide solution at a mass ratio of 1:12, add to plasma reaction vessel, set plasma power to 550W, and continue treatment for 20min. Filter and dry to obtain LiMnFePO4-COOH for later use.

[0093] S2. Add 30g LiMnFePO4-COOH to an aqueous solution (450mL) containing 20g ethylene glycol, add 50mL of 1mol / L ferric chloride solution, add ammonia water dropwise and react for 2h, centrifuge to separate and dry, and finally calcine at 210℃ under an argon atmosphere for 1.5h to obtain LiMnFePO4@Fe3O4.

[0094] S3. Mix LiMnFePO4@Fe3O4 with a 1% graphene oxide solution at a mass ratio of 1:1, stir in a water bath at 45℃ for 5 hours, centrifuge after the reaction is completed, dry in an oven at 60℃ for 24 hours, and then calcine (keep warm in a nitrogen atmosphere at 220℃ for 1.5 hours) to obtain the positive electrode active material.

[0095] Table 1 Table 2 Performance testing Recovery amount: The mass of the positive electrode active material assembled into a secondary battery is taken as m1. After the secondary battery is cycled to 80% SOH, the secondary battery is disassembled, the positive electrode plate is removed, scraped off, ground with a mortar and pestle, and then the positive electrode active material is attracted out with a magnet. The mass of this positive electrode active material is taken as m2. Recovery amount ratio = m2 / m1 × 100%.

[0096] Gas production performance test: The secondary battery was placed in a charge / discharge test chamber at a constant temperature of 25°C, with a voltage range of 2.5~4.25V, a charge rate of 1C, and a discharge rate of 1C, and cyclic testing was performed. The test was stopped after 100 cycles. The gas production volume of the secondary battery was tested using the water displacement method.

[0097] Rate performance test: The secondary battery that has not undergone electrochemical testing is placed on a charge-discharge tester at room temperature. It is charged at 0.33C to 4.25V and discharged at 0.33C to 2.5V, with a capacity of Q2. It is charged at 0.33C to 4.25V and discharged at 5C to 2.5V, with a capacity of Q3. Rate performance = Q3 / Q2 × 100%.

[0098] Cyclic performance test: The secondary battery is placed in a charge / discharge test chamber at a constant temperature of 25°C, with a voltage range of 2.5~4.35V, a charge rate of 1C, and a discharge rate of 1C for cyclic testing. The test is stopped when the capacity drops to 80% of the initial capacity, and the number of cycles is recorded.

[0099] Table 3 As can be seen from Table 3, when the technical solution provided in this application is adopted, the obtained secondary battery has excellent rate performance, cycle performance, high recovery volume, and low gas production; specifically, the rate performance of the obtained product is above 83%, the number of cycles is above 2120, the gas production volume is below 8.0 mL, and the recovery volume is above 92%. As can be seen from the examples and comparative examples 1 to 3, the positive electrode active material structure provided in this application can achieve significant effects.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material includes a core, a first shell layer disposed on at least a portion of the surface of the core, and a second shell layer disposed on at least a portion of the surface of the first shell layer; The core comprises a magnetic oxide, the first shell comprises at least one of lithium transition metal phosphate and / or lithium transition metal oxide, and the second shell comprises a carbon material.

2. The positive electrode active material according to claim 1, characterized in that, Satisfy at least one of the following (A) to (D): (A) The magnetic oxide includes at least one of iron(II,III) oxide, iron(III) oxide, iron-silicon alloy, and iron-nickel alloy; (B) The lithium transition metal phosphate includes at least one of lithium iron phosphate and lithium manganese iron phosphate; (C) The carbon material includes at least one of hard carbon, soft carbon, graphene oxide, reduced graphene, and carbon nanotubes; (D) The lithium transition metal oxide includes at least one of lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide.

3. The positive electrode active material according to claim 1, characterized in that, Satisfy at least one of the following (a) to (c): (a) The average particle size of the magnetic oxide is 30~200 nm; (b) The thickness of the first shell layer is 100~2000 nm; (c) The thickness of the second shell is 3~30nm.

4. The positive electrode active material according to claim 1, characterized in that, The carbon material satisfies: 0.8 ≤ I d / I g ≤2; I d The intensity of peak D in the Raman spectrum of the carbon material; I g The intensity of the G peak in the Raman spectrum of the carbon material is given.

5. The positive electrode active material according to claim 1, characterized in that, The specific surface area of ​​the positive electrode active material is 30~260m². 2 / g; And / or, the particle size Dv50 of the positive electrode active material is 100~2230nm.

6. The positive electrode active material according to claim 1, characterized in that, The carbon material also includes element M, which includes at least one of N, O, F, Cu, Ti, Co, and Ni.

7. The positive electrode active material according to claim 6, characterized in that, The mass percentage of element M in the carbon material is 0.1-3%.

8. The positive electrode active material according to claim 1, characterized in that, Based on the mass content of the positive electrode active material, the magnetic oxide accounts for 5-25%, the lithium transition metal phosphate and / or lithium transition metal oxide accounts for 75-95%, and the carbon material accounts for 0.1-3%.

9. A secondary battery, characterized in that, Includes the positive electrode active material as described in any one of claims 1 to 8.

10. An electrical device, characterized in that, The device includes the secondary battery as described in claim 9, wherein the secondary battery serves as the power supply for the electrical device.

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